1. Field of the Invention
The invention may be used for forming conductive tracks in electronics, microelectronics, and for switching electronic circuits and semiconductor devices.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Conductive tracks are conductive metallization patterns, mainly used on non-conductive ceramic substrates. Most frequently, such conductive patterns consist (see FIG. 1) of a layer of a conductive metal, for example, copper. In some cases, in order to improve adhesion of such a layer to the substrate surface, a thin adhesion layer is positioned therebetween. In order to protect copper against oxidation and copper migration, a copper layer is covered with a barrier layer, for example, a nickel barrier layer. Since such tracks are subsequently subjected to soldering and micro-welding, an upper layer is applied thereon, which may consist of gold (for soldering, welding) or tin (for soldering).
The following methods for producing conductive patterns are known in the art.
Thick-film technology. The most conventional and cheap technology is the so-called “thick-film” technology [1] that comprises application of special metal-containing pastes onto ceramics through a template with subsequent paste co-firing with the ceramics at a high temperature and simultaneous removal of binders and formation of a metal layer (FIG. 2).
This method does not require expensive equipment and exhibits high productivity. A cost of this process for a single product is the lowest among all the methods applied. This method finds wide use due to its cheapness, a short production cycle and high productivity.
Its drawbacks are:                low resolution of such metallization patterns. This is associated both with limitation of templates as to accuracy, and with inevitable dithering of a pattern during co-firing. Resolution ranging from 0.1 to 0.2 mm is insufficient for microelectronic production sites wherein high resolution is required, better in the range from 0.05 to 0.02 mm;        palladium silver-containing pastes are conventionally used for creating patterns. These pastes have comparatively low co-firing temperatures. But the negative fact is the presence of silver. Silver is an impurity with abnormal mobility, and this impurity diffuses, thus impairing parameters of devices having such a pattern. More acceptable are copper-containing pastes that have a higher temperature of the co-firing process. Substitution of more advantageous copper pastes for silver-containing ones requires higher co-firing temperatures in ovens and applying vacuum. This increases the process costs significantly;        poor controllability of metallization thickness. Non-ideal surfaces of metal patterns having sloped edges and high roughness;        substrate types suitable for co-firing with metal-containing pastes are limited.        
Thin-film technology. The so-called thin-film technology used for creating a metallization pattern is most suitable for producing high-resolution metallization patterns. This technology is typically used in the microelectronic industry for producing contact layers, e.g., in the production of UHF instruments [2].
The method for creating a conductive pattern comprises vacuum deposition of an adhesive metal layer (chrome, nichrome, tantalum, etc.), deposition of a thin copper layer, photolithographic creation of a pattern (application of a photoresist, its exposure and development), additional galvanic growth of the copper layer to a required thickness and a nickel barrier layer. Then, the photoresist layer is removed, and the continuous layer is etched. A layer of gold or tin (containing bismuth) is applied over the conductive layer for solderability.
The method is ideal for producing high-resolution metallization patterns and controlling layer thicknesses.
The main drawbacks of the method result from many stages of the process and its long cycle, the stages requiring complex and high-precision equipment:                multi-stage process;        high-precision and expensive equipment.        
A metallization pattern itself has several drawbacks, namely:                a galvanically produced conductive layer has poorer properties as compared to a voluminous material or layers produced by vacuum deposition;        the surface of a galvanically produced layer has increased roughness;        due to the technology used, underetching is inevitable in the base of pattern elements, and the protective layer does not fully covers the conductive layer (FIG. 3).        
DBC technology. Stands for “Direct Bonded Copper” [3] and means creation of a metallization pattern in a copper layer in the form of a thin foil that is co-fired with a ceramic substrate.
The surface of a ceramic substrate is covered with a thin sheet of a foil. Then, copper is sintered with the substrate by applying pressure and temperature.
Then, a pattern is produced on the copper surface by using the technology similar to production of PCBs, namely, photolithography with subsequent etching of the pattern (FIG. 4).
The drawbacks are:                due to the technological limitations, thicknesses of such a copper pattern may not be less than 100-125 microns. It is a very big copper thickness for subsequent creation of a metallization pattern having a quality resolution. A significantly lower thickness—15-40 microns, not more—is required for many applications;        the cost of such a product is very high, significantly higher than that of a heat-transfer in the thin-film technology.        